added option to change History.squeeze behavior

We currently squeeze multiple History columns into one by choosing the last column. This makes sense in many cases, but, as I got a request for alternative aggregation schemes from the IAGOS-project, obviously not in all. This MR implements the option to select from several squeezing options, currently "last" (out current implementation), "min" (select the smallest flag per row), "max" (select the highest flag per row).

As I didn't really find a good place, where this rather special behavior, that furthermore requires quite some insight into the saqc-machinery, should be controlled, it is now/currently set by a module global variable.

Any thoughts @palmb and @luenensc ?

tests/core/test_history.py

@@ -253,6 +253,7 @@ def test_append_force(__hist, s, max_val):
 )
 def test_aggregations(__hist, col, expected, hist=History(index=pd.Index(range(6)))):
     import saqc.core.history
+
     hist.append(col)
     check_invariants(hist)
     for aggregation in ["last", "min", "max"]: